VARIABLE-TEMPERATURE SOLID-STATE C-13 NMR-STUDIES OF NASCENT AND MELT-CRYSTALLIZED POLYETHYLENE

Nascent and melt-crystallized ultrahigh molecular weight polyethylene (PE) have been studied using solid-state C-13 NMR spectroscopy. Signif icant differences between the two materials are observed in the CPMAS spectra particularly at elevated temperatures. Three separate resonanc es are observed in nascent samples when the temperature is raised to n ear the melting point, the third peak being of an intermediate chemica l shift between the crystalline and amorphous resonances that are obse rved in the spectra of melt-crystallized samples. Based on this chemic al shift, the third resonance is likely due to polymer chains which un dergo conformational averaging which is intermediate between the rigid all-trans crystalline polymer and very mobile amorphous chains. Evide nce is reported which shows that this is material that is present in n ascent PE below the melting point and is not created during the meltin g process. These observations are confirmed by the measurement of 2D w ide-line separation (WISE) experiments where the H-1 line width of the amorphous component of melt-crystallized PE is much narrower than tha t of a nascent sample. Numerous nascent PE samples, including those of low molecular weight, display similar spectral features.